A. Technical Field
The present invention relates to a method for treating an exhaust gas, which can effectively treat an exhaust gas also containing metal mercury together with a nitrogen oxide.
B. Background Art
Conventionally, as a method for treating a nitrogen oxide (NOx) in an exhaust gas, there is known a selective catalyst reducing method (so-called SCR method) of catalytically reducing a nitrogen oxide in an exhaust gas on a denitration catalyst using a reducing agent such as ammonia and urea, to decompose a nitrogen oxide into harmless nitrogen and water, and an exhaust gas treating system adopting this has been put into practice.
However, mercury which is a harmful substance likewise a nitrogen oxide (NOx) is contained as metal mercury (Hg0) or mercury halide such as mercury chloride (HgCl2) in an exhaust gas in some cases.
Since when mercury is present as mercury halide, it is easily absorbed into water, it is relatively easy to capture and remove mercury. However, since when mercury is present as metal mercury, it is hardly absorbed in water, it is difficult to remove mercury.
As the technique of removing mercury (including metal mercury) from an exhaust gas, with consideration given to such behavior of the mercury, there have hitherto been known, for example, an active carbon adsorption method and a sodium hypochlorite absorption method (for example, see patent documents 1 and 2 below). More particularly, as an active carbon adsorption method, for example, a method of blowing an active carbon powder into an exhaust gas, and recovering mercury with a bag filter has been put into practice. On the other hand, as a sodium hypochlorite absorption method, for example, a method of directly adding sodium hypochlorite to cooling water in a cooling tower in an exhaust gas treating system, an absorbing solution of a desulfurization absorption tower, supply water of a wet electrostatic precipitator or circulating water has been put into practice.
However, in the active carbon adsorption method, since active carbon with mercury adsorbed thereon cannot be used repeatedly by regenerating the active carbon, substantial cost is required for active carbon and, moreover, disposition of used active carbon becomes a problem. On the other hand, since the sodium hypochlorite absorption method adds sodium hypochlorite to a main equipment of an exhaust gas treating system, corrosion in an apparatus is feared and, at the same time, substantial cost is required for sodium hypochlorite and, moreover, there is a possibility that secondary environmental pollution due to the sodium hypochlorite, and disposition of produced waste water also become a problem. For this reason, both of the active carbon adsorption method and the sodium hypochlorite absorption method are put into practice only in treatment of a small amount of an exhaust gas such as a trash incineration exhaust gas and, thus, it is difficult to apply them to treatment of a large volume of a gas such as an electric power station.
Therefore, there is demanded a new method which can be also applied to treatment of a large volume of an exhaust gas without using active carbon or sodium hypochlorite for the purpose of removing mercury (including metal mercury), and can effectively treat metal mercury together with a nitrogen oxide.                [Patent Document 1] JP-A-031323/1993 (Kokai)        [Patent Document 2] JP-B-046270/1972 (Kokoku)        